The magnetic tape industry is continually searching for better methods to produce magnetic media useful for specific end-use applications, such as audio, video, and computer information storage and retrieval applications. The conventional processing technique employed to produce these magnetic media generally includes dispersing magnetic pigment (usually acicular magnetic particles) in a binder resin/organic solvent solution, coating this liquid dispersion on a tape substrate, aligning the magnetic particles by passing the coated tape through a magnetic field of sufficient strength, and then calendaring the coated tape to achieve the desired surface properties. The resulting magnetic media thus generally consists of a nonmagnetic substrate and an overlying magnetic layer or film principally made of the magnetic particles and a resin binder matrix.
The organic solvents usually employed during the production of conventional magnetic media (e.g., methyl ethyl ketone, methyl isobutyl ketone, and the like) are, however, environmentally and toxicologically hazardous. This necessitates that the organic solvent be recovered completely during the production of magnetic media--a costly operation. It would therefore particularly be advantageous if an organic solvent system was employed which includes at least a meaningful amount of a non-hazardous organic substance. This would, therefore, reduce (if not eliminate) the costs associated with solvent recovery. And, it would also be desirable if a solvent system was available which would serve dual functions--that is, a system which would not only serve as a solvent for the binder resin, but which would also be polymerizable in situ on the substrate surface and thus remain as a component of the binder matrix, thereby obviating the need for its recovery. It is towards achieving these advantages that the present invention is directed.
According to the present invention, a magnetic dispersion is provided which includes a binder resin, magnetic particles, and an organic solvent system for the binder resin which includes relatively non-hazardous trioxane. The trioxane may thus be present as the sole solvent, or as a cosolvent with one or more organic solvents, thereby at least reducing (if not eliminating) the amount of potentially hazardous organic solvent needed to form magnetic coatings during the production of magnetic media. When used in this manner, the trioxane is allowed to sublime before it crystallizes (which may be facilitated by heating the substrate and/or the liquid dispersion to a temperature above the crystallization temperature of trioxane).
The trioxane may also be provided as a reactive solvent. That is, a suitable tioxane-polymerizing initiator (or polymerization initiator/catalyst system) may be incorporated into the liquid dispersions of this invention so that the trioxane is capable of being polymerized to poly(oxymethylene) in situ on the non-magnetic substrate. The resulting trioxane polymerization product (i.e., poly(oxymethylene)) is therefore present as a resin alloy in the binder resin matrix on the non-magnetic substrate.
Other advantages of this invention will become more clear after careful consideration is given to the following detailed description thereof.